Not applicable.
Not applicable.
The present invention relates to transformers for converting three-phase power to nine-phase power, and more particularly to transformers for providing reduced harmonics on the AC and minimizing ripple on the DC side of an AC to DC rectifier.
Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch-based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion that, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three-phase and therefore three-phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. As with most hardware intensive configurations cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases that are rectified by the rectifier. These AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase harmonics and ripple are reduced appreciably. Where AC harmonic restrictions are placed on rectifier systems such restrictions are often satisfied by employing an 18-pulse rectifier that requires a nine-phase source of AC power. As the global standard for AC power distribution is three-phase, 18-pulse rectifiers require three-to-nine phase power converters between utility supply lines and rectifier switches.
Isolation transformers for converting three-phase AC power to nine-phase AC power are known in the art but have several shortcomings. First isolation transformers must be rated for the full power required. Second, isolation transformers are typically relatively large as separate primary and secondary windings are required for isolation purposes.
Where isolation between a utility supply and a rectifier is not required, employing an autotransformer including a plurality of series and common windings may advantageously reduce the size and weight of a three-to-nine phase converter that consists of an autotransformer and a rectifier unit. Exemplary three-to-nine phase autotransformers are described in U.S. Pat. No. 4,876,634 (the xe2x80x9c""634 patentxe2x80x9d); U.S. Pat. No. 5,124,904 (the xe2x80x9c""904 patentxe2x80x9d); U.S. Pat. No. 5,619,407 (the xe2x80x9c""407 patentxe2x80x9d); and U.S. Pat. No. 5,455,759 (the xe2x80x9c""759 patentxe2x80x9d), each of which is incorporated herein for the purpose of describing the prior art.
The ""634 patent teaches the general concept of providing three-phase autotransformer coils in a plurality of series connected windings which are arranged to form a hexagon. Three-phase AC input lines are linked to three input nodes and nine output nodes provide voltages to three rectifier bridges. Phase shift between the output voltages is accomplished by providing long and short windings between the input nodes and the output nodes. Importantly, the ""634 patent teaches that, for each autotransformer input phase, the phase shift between three corresponding output voltages should be 20 degrees and accomplishes 20 degree phase shift by providing short windings between each two adjacent output nodes corresponding to the same input phase. Long windings are provided between adjacent output nodes corresponding to different input phases. In the ""634 patent the nine output voltages are provided to three separate six-pulse bridges.
Unfortunately, there are at least two problems with the 18-pulse autotransformer described in the ""634 patent (hereinafter the ""634 topology). First, there is an inherent impedance mismatch in the ""634 topology which results in looping currents among the three bridges and which requires additional hardware to correct. For example, when the outputs and inputs to the ""634 18-pulse autotransformer are linked to provide unity gain one of the three bridges is fed directly from the input power source while the other two bridges are fed through transformer windings which each are characterized by a certain amount of leakage inductance. This means that there are different impedances for each of the bridges and the different impedances cause disparate DC output voltages and hence looping currents among the bridges. A similar impedance disparity results when the ""634 patent 18-pulse autotransformer is linked for step-down transformation.
The ""634 topology attempts to use two inter-phase transformers to reduce the looping currents. As an initial matter Applicant believes the inter-phase transformers provided in the ""634 topology are erroneously specified and that six, not two, inter-phase transformers would be required to reduce the looping currents. While six inter-phase transformers can be provided, inter-phase transformers are required to carry DC bus currents. Therefore, inter-phase transformers are relatively bulky and increase system size appreciably. In addition, the six inter-phase transformers are relatively expensive and increase system costs.
Second, the ""634 topology would result in current sharing problems among the three bridges due to enclosed electrical circuits formed by the multi-phase shift bridges. The current sharing problems are exacerbated when AC line harmonics occur as different source harmonics substantially change bridge current sharing. Because AC line harmonics are often irregular and unpredictable it is impossible to balance the impedance mismatch via addition of resistance elements. While the inter-phase transformers may ease current harmonics to the power source, the inter-phase transformers are not effective as a solution for the current sharing problem.
Because of the current sharing problem described above all three bridges in the ""634 topology have to be capable of handling over-rated current conditions as high as 150% of the current level required to be handled if the bridges were able to share current equally. This is because from time to time each bridge is forced to operate close to its rated current level while the other bridges only operate at 50% of their rated level. This drastic current difference among bridges also forces the windings of the ""634 topology to carry appreciably disparate current magnitudes. For this reason, in addition to the bridges having high current ratings, the autotransformer also must be rated to handle high current value and therefore results in inefficient material utilization.
One solution to the looping and sharing current problems associated with the ""634 topology is to provide an autotransformer that equally spaces output voltages in phase. For example, where there are nine outputs the outputs can be phase shifted from each other by 40 degrees each. In the ""407 patent this is accomplished by providing an autotransformer having three coils, each coil having a plurality of serial windings and a plurality of stub windings. The serial windings form a delta and the stub windings are magnetically coupled with the serial windings from the same coil. Three terminals are provided as the apices of the delta and the three-phase AC inputs are linked to the apex terminals. A plurality of direct outputs is interposed between respective serial windings and a plurality of indirect outputs is electrically connected with the second ends of the stub windings. The windings are chosen such that the voltage magnitudes of the direct and indirect outputs are identical. Other autotransformer topologies which include stub windings are described in the ""904 patent and the ""759 patent.
While staggering the transformer outputs by 40 degrees essentially eliminates the looping and sharing current problems identified above, the stub winding requirement in each of the ""407, ""904 and ""759 patents renders those solutions wasteful of winding and core material.
In addition to the problems discussed above, often specific DC loads require different DC magnitudes. For example, in some cases a DC load may require a DC magnitude that is essentially identical to the AC supply magnitude and in other cases a DC load may require a stepped down DC magnitude that is less than the supply AC magnitude. One solution to this dilemma is to manufacture two different transformers, a step-down transformer and a step-up transformer. This solution, however, is relatively expensive as two designs are required and the expenses associated with manufacturing two different transformer designs can be appreciable.
Despite the relatively large size of isolation transformers, sometimes specific applications require isolated primary and secondary windings. In the isolated transformer topologies many of the same design concerns have to be considered. For example, isolation transformers should be designed so as to minimize input current harmonics, minimize DC bus voltage ripple, eliminate bus current sharing problems, reduce overall transformer size and minimize required materials thereby reducing cost.
Thus, it would be advantageous to have a three-to-nine phase transformer that did not cause looping and sharing current problems and that is relatively inexpensive to construct. In the case of an autotransformer it would be advantageous if the transformer could be used either as a unity gain or a step-down transformer.
The present invention includes an autotransformer for transforming three-phase AC input voltages to nine-phase AC output voltages wherein the transformer includes three coils, each coil forming a plurality of series windings, the windings arranged to form a polygon. Nodes between the windings form nine output nodes, at least one step-down input set (i.e., three step-down input nodes) and at least one unity gain input set (i.e., three unity gain input nodes). The windings are sized and configured such that the voltage magnitudes at the output nodes are identical, the voltage magnitudes of the step-down input set are identical, the voltage magnitudes of the unity gain input set are identical, the unity gain set voltage magnitudes are identical to the output node voltage magnitudes, the step-down input set voltage magnitudes are greater than the output node voltage magnitudes, adjacent output nodes are separated by 40 degree phase shifts and the nodes in each input set are 120 degrees out of phase (i.e., the second node in each input set is 120 degrees out of phase with the first and second nodes in the set and the third node is 120 degrees out of phase with the first node in the set).
Thus, one object is to provide an autotransformer that avoids the looping and current sharing problems discussed above. To this end, because the output voltages have identical magnitudes and are equi-phase-shifted (i.e., adjacent output node voltages are 40 degrees out of phase), looping and current sharing problems are essentially eliminated.
Another object is to achieve the aforementioned object relatively inexpensively. To this end the present autotransformer only includes serial windings and does not require stub windings. Thus, less winding material is required to provide the desired transformation results.
One other object is to provide a step-down transformer that avoids the looping and current sharing problems. To provide a step-down autotransformer the present configuration provides the step-down input set of input nodes where the magnitudes of voltages at the step-down input set are greater than the magnitudes at the output nodes.
Yet another object is to provide a single autotransformer that can be used either as a step-down transformer or as a unity gain transformer. This feature enables a manufacturer to provide a single transformer that can be used in two different applications and therefore reduces design and manufacturing costs as only a single autotransformer has to be designed and manufactured instead of two different autotransformers. To this end the preferred autotransformer configuration provides both the step-down input set and the unity gain input set.
The invention also includes an isolation transformer having three input lines linked to a primary and nine outputs linked to a secondary. The primary may be in either a delta or a Wye form including three primary windings separated by 120xc2x0 phase shift. The secondary preferably has the same form as the three-phase autotransformer configuration described above with the nine outputs linked to the secondary as described. In this manner the isolation transformer achieves many of the same advantages achieved with the inventive autotransformer. In addition, as the name implies, the isolation transformer isolates the primary and secondary windings. Furthermore, the isolation transformer may be configured such that the primary windings are tap-adjustable so that the transformer turns-ratio can be adjusted xe2x80x9cin the fieldxe2x80x9d to cause step-up, step-down or unity gain to accommodate field source characteristics and facilitate optimal system operation.
A complete understanding of the present invention will be obtained from the following description and the accompanying figures.